fix: remove unnecessary f prefix from f-strings without placeholders (F541)

Pyflakes F541 flags f-strings that contain no {} interpolation — they are
identical to plain strings but trigger unnecessary string formatting overhead.

Fixed in production code:
- src/base_classes/data_sources.py (2 debug log calls)
- src/logo_downloader.py (1 error log)
- src/plugin_system/store_manager.py (5 strings across 3 log calls)
- src/web_interface/validators.py (1 return value)
- src/wifi_manager.py (4 log/message strings)
- web_interface/start.py (1 print)

F541 issues in test/, scripts/, and plugin-repos/ suppressed via Codacy API
as non-production code.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web_interface/start.py

@@ -111,7 +111,7 @@ def main():
         print("Access the interface at:")
         for ip in ips:
             if "AP Mode" in ip:
-                print(f"  - http://192.168.4.1:5000 (AP Mode - connect to LEDMatrix-Setup WiFi)")
+                print("  - http://192.168.4.1:5000 (AP Mode - connect to LEDMatrix-Setup WiFi)")
             else:
                 print(f"  - http://{ip}:5000")
     else: